A ship provided with a pod propulsion unit is known. The pod propulsion unit is provided with a pod and a propeller which is driven by a propeller driving mechanism disposed in the pod. A propulsion function is achieved by the propeller. A steering function is achieved by changing the orientation of the pod propulsion unit by a steering mechanism disposed in the hull.
Patent Literature 1 discloses a ship provided with a pod propulsion unit. The pod propulsion unit is installed to a hull shell plate in the rear portion of the hull through a strut to be rotatable around a perpendicular axis. This ship is provided with a steering unit which is independent from the pod propulsion unit. The rudder plate of the independent steering unit is arranged behind the strut and is installed to the hull to be rotatable around a perpendicular axis. The rudder plate can be rotated without an interference that strut and the rudder collide.
The pod propulsion unit is not operated or is operated by a small rudder angle at the time of a steering operation test in a test sailing, a high-speed sailing, or a large rudder angle sailing for urgent avoidance, and the rudder plate of the independent steering unit is operated to a necessary rudder angle, e.g. a maximum rudder angle to generate enough rudder force. Because the rudder plate is arranged behind the strut in neighborhood to the strut, the large rudder force can be obtained by using the interference with a flow of water. Because the pod propulsion unit is not operated or is operated by the small rudder angle, there is no case that the large force acts on the pod propulsion unit, and furthermore damage due to erosion can be prevented. Note that Patent Literature 1 does not disclose that a contrarotating propeller is configured from the propeller of the pod propulsion unit and another propeller.
Patent Literature 2 discloses a ship that a main propeller and a propeller of a pod type propulsion unit configure a contrarotating propeller. The pod type propulsion unit has a support axis installed to the hull to be rotatable, a body section installed to the lower end of the support axis, a fin installed under the body section, and a propeller rotatably driven by a motor built in the body section. The support axis is rotatably driven by a motor disposed in the hull. In the pod type promotion apparatus, both of the support axis and the fin function as rudders. In this ship, two rudders are disposed for both sides of the pod type propulsion unit. The two rudders are arranged in middle positions between the centerline of the hull and the sides of the ship. Because the ship has the pod type propulsion unit with the support axis to achieve the function of the rudder, the ship does not originally have to provide the two rudders. However, in a high-speed sailing, cavitation occurs in the neighborhood of the support axis. Because this cavitation is remarkable as the rudder angle is large, the rudder angle of the support axis is limited to an angle within a predetermined angle range in the high-speed sailing. Thus, because it brought about hinderance to the sailing of the ship, the two rudders are provided.
Patent Literature 2 discloses another ship in which the main propeller and the propeller of the pod type propulsion unit configure the contrarotating propeller. In the other ship, one rudder is installed behind the pod type propulsion unit. Because it is sufficient to provide a single rudder, an installation cost is reduced, compared with a case to install two rudders. In addition, because the rudder is provided in a position where the rudder interacts with a water flow generated by the main propeller and the propeller of the pod type propulsion unit, an enough steering ability can be secured even if the rudder is singular.
Patent Literature 3 discloses a ship in which a main propeller, a pod propulsion unit, and a rudder are provided in order from the bow side to be aligned on a keel line.